Fluid catalytic cracking (FCC) is a main process for obtaining highly ranked petroleum related products, such as gasoline, diesel oil and liquid petroleum gas (LPG), from heavy feeds having usable light fractions. The feeds most often submitted to the FCC process are generally those refinery streams that have origin in side cuts of vacuum towers, called heavy vacuum gasoil, or heavier streams that find origin in the bottom of atmospheric towers called atmospheric residue or even a mixture of those streams.
Such streams, when submitted to the FCC process, are contacted with a catalyst made up of a fine particulate material in a conversion zone in the absence of hydrogen and are converted in lighter and more valuable hydrocarbon streams, separated from streams that are even heavier than the feed.
In spite of the fact that the FCC process is more than 50 years old, techniques that might improve the process are continuously sought, increasing the yield in products of higher intrinsic value. Generally, it is agreed that the main goal of the FCC processes is the maximization of the production of higher intrinsic value products.
One relevant aspect of the process is the initial contact of the catalyst with the feed; that is, the interaction promoted by the above cited dispersion systems has a marked influence on the conversion and selectivity to valuable products.
A few trials aiming at improving the contact between the catalyst and the feed have been carried out, always based on the idea of promoting a quick vaporization of the feed as well as an intimate contact with the catalyst during the small period of time available within the riser. In order to process the catalytic cracking reactions it is required that the vaporization of the feed in the area of mixture with the catalyst occurs within a few milliseconds so that the molecules of the vaporized hydrocarbons may contact the catalyst particles, permeating through the catalyst macropores and suffering the effect of the acid sites that promote the catalytic cracking. If a quick vaporization does not occur, the result is a thermal cracking of the still liquid fractions.
It is well known that the thermal cracking leads to the formation of by-products such as coke and fuel gas, mainly in the case of residuum-containing charges. Therefore, the cracking on the riser bottom undesirably competes with the catalytic cracking that is the object of the FCC process.
One important parameter for the feed atomization is its temperature in the atomizer. Some of its physical properties such as viscosity and surface tension are altered as a function of temperature and during the atomization process result in a universe of lower diameter droplets. Therefore a substantial increase in the contact area by the surfaces of the droplets present in the spray occurs, this entailing a significant impact on the ease of vaporization. For residual feeds used in the FCC process and at the recommended temperature ranges, it may be demonstrated that the increase in contact area by using higher feed temperatures may attain 30%. However the feed temperature cannot be indefinitely increased since there is the risk of coke build up and non-selective thermal cracking within the feed furnaces.
On the other hand the quick vaporization of the feed will be obtained more easily if the feed is suitably atomized, so as to form a thin spray on the catalyst phase. In order to obtain that spray several models of feed injectors in the riser have been developed.
According to one of the first of such developments, the feed and the steam were added to the catalyst from the regenerator with the aid of a Y tube, in a system known as “Y jet”, which in practical terms nearly did not disperse the feed, leaving to the hot catalyst the transfer of heat to the feed and the subsequent vaporization. This model was acceptable for lighter feeds where the vaporization caused by the heat transferred by the catalyst was practically instantaneous.
Since the eighties, with the advent of heavier feeds from heavier oils in the FCC units, several modifications were introduced in the feed injection system. One of such changes has been the replacement of the so called single feed-dispersion system by multiple feed-dispersion system, placed at elevations between 30° and 70°, at one or more levels, so as to provide a better feed dispersion as well as a better contact with the catalyst. The standard flat spray was at first widely used for this purpose.
Other kinds of feed-dispersion systems have been developed concomitant to the increase in the severity of the feeds to be cracked.
U.S. Pat. No. 4,434,049 teaches the atomization of a water/oil emulsion by a feed-dispersion system the feature of which is the modification of the size of the oil particles by the impact of the emulsified feed against a flat cylindrical surface. As alleged by the authors, the feed-dispersion system produces a spray having oil particles of circa 500 micra diameter that are then accelerated by the steam entering by a spot perpendicular to the feed inlet. The inlet rate of steam causes that the oil particles are submitted to shear, this rendering such particles still smaller; the mixture of steam and emulsified feed is accelerated up to an outlet nozzle having a special geometry so as to obtain the feed dispersed as a fine spray. However, the described device requires that the feed be introduced as an emulsion with water so that the surface tension is reduced, and then the water/oil micelles are broken by the impact against the flat cylindrical surface.
European patent EP 546739 relates also to a device for the feed injection that uses the principle of breaking oil particles through the collision with a flat surface, without however requiring the previous emulsification of the oil with water.
Brazilian PI BR 8404755 teaches a feed-injection device where the feed and the atomizing fluid -steam- are admixed within a chamber in order to promote the dispersion of the feed in an efficient way. The mixing chamber bears a central pin the diameter of which controls the flow rates in the annular space. The atomizing fluid, distributed through several holes, enters perpendicularly to the feed. A mist is then formed that is directed to the interior of the riser.
U.S. Pat. No. 5,037,616 (corresponding to European EP 312428) teaches that a good dispersion of the feed with vapor may be obtained with the aid of a feed injector featured by a venturi tube. Dimensions characterize the geometry of this device such that the speed of the feed and steam mixture reaches sonic conditions at the venturi throat. On its turn, the venturi tube shows a cylindrical internal section and is situated between the converging and diverging sections. The continuity of the converging, cylindrical and diverging sections is smoothly made by means of a curved section. The superior angle of the device with the venturi tube is around 5° to 15° and the diameter of an exit hole is at most 2 to 5 times the venturi tube diameter. In the average, oil droplets having diameters of the order of 10 to 50 micra are formed, that are injected in the riser at speeds of the order of 60 to 150 meters by second.
U.S. Pat. No. 5,173,175 teaches a device for feed injection into a fluid catalytic cracking reaction zone, the device comprising a straight section where the feed and steam are pre-mixed and a terminal section where oil is atomized and dispersed by means of a fan-like distributor. The feed injector yields a flat vaporization standard that is perpendicular to the catalyst flow direction in the contact section between the catalyst and the oil in the cracking zone. It is alleged that better product yield and less coke and gas are produced. The system described in said US patent works so that the fluids are admixed prior to the element that promotes the feed atomization and causes the fan-like jet formation. On the contrary, in the present application the fluids are admixed exactly on the bottom of the device that promotes the atomization and the formation of the fan-like flat jet. The atomization is promoted by the efficient contact between the steam from the atomizing fluid nozzle (the fluid being generally steam) and the couple of charge nozzles that surround the atomizing nozzle.
Besides, the working condition described in U.S. Pat. No. 5,173,175 as well as in all documents where the technique employs the previous mixture of the feed and the atomization fluid causes the following feature linked to the loss of charge (or ΔP to conform to the widely spread jargon). The previous mixture makes that the loss of charge between the interior of the riser where the charge jet and atomizing fluid is admixed to the catalyst is shared by both fluids, charge and atomizing fluid. Common charge loss implies that a considerable portion of the energy of the atomization fluid is not used for promoting the atomization.
U.S. Pat. No. 5,673,859 teaches a vaporization nozzle for fluid catalytic cracking that shows two discharge orifices elongated in the cross direction to effect a fine atomization of the liquid hydrocarbon charge as said charge is vaporized by the nozzle. Preferably the orifices are inclined so as to produce a convergent spray but may be inclined to produce a divergent spray or a substantially flat spray. The basic principle of said system is to use the dissipation of kinetic energy of the charge through the inelastic shock with metal bar 13 to promote atomization. Thus, to obtain good atomization a high pressure upstream of device 15 is required. Due to the reduction to the square in kinetic energy with feed flow rate, by working with reduced feed flow rates the atomization performance would be seriously jeopardized. On the contrary, in the present application this effect does not exist since the atomization energy is independent of the charge flow rate.
U.S. Pat. No. 5,794,857 corresponding to PI BR 9607665-8A, teaches a device for feed injection mounted with two concentric conduits where the inner conduit is the steam conduit and the outer conduit, the feed conduit, so that both conduits define an annular liquid conduit for the feed. The outlet end of the inner conduit is semi-spherical and has a row comprising a plurality of holes therein for the passage of the steam; the also semi-spherical outlet end of the outer conduit has an elongated slit parallel to the orifices of the semi-spherical outlet of the inner conduit for passage of steam and feed as a spray. It is alleged that the device allows for the operation at low steam pressure, or even in the absence of steam in case any problem occurs caused by the refinery steam feed. Contrary to the technique taught in the said US patent, in the present application the energy of the atomization fluid is transformed in a more efficient way using a converging section having a variable converging angle so as to make an efficient conversion of the atomization fluid pressure into kinetic energy and promoting the feed atomization. The contact of the feed with the atomization fluid is carried out by means of nozzles that direct the contact of the feed with steam so that the generated kinetic energy is transmitted to the feed, instantaneous and intense atomization being promoted.
Therefore, the patent literature does not teach nor suggest the purpose of the present invention, that is, a feed-dispersion system whose geometry is able to promote the atomization of the feed so that the average diameter of the oil particles is in the range of 100 micra, with the improved use of the transfer of the atomization fluid energy to the feed. This way, a better performance of the process and the catalytic cracking fluid unit is made possible.